1. Field of the Invention
The present invention relates to electronic power supplies and specifically to switching power supplies that frequently encounter no load conditions and must be as efficient as possible to extend battery life and to reduce heat dissipation.
2. Description of the Prior Art
Two types of power supplies are in wide use that convert an incoming source to voltage levels useful to the equipment being powered. Linear power supplies are the simplest a series transistor typically drops enough voltage across it to maintain a constant output voltage. A problem with linear power supplies is that the heavier the output load, the more current will have to pass through the regulating transistor, since it is in series with the load. This increased current means more power must be dissipated by the transistor, and this power is thrown away as heat. Switching power supplies are more efficient and run cooler than linear power supplies at the same power levels. But switching power supplies are more complicated because a transistor operated as a switch is used to chop incoming current for a transformer that has the load connected to its secondary winding, usually through a rectifier and filter. A feedback circuit is needed to monitor the output voltage and increase the time duration the chopping transistor is on when more output voltage is needed and to reduce the time duration when less output voltage is needed. The feedback circuit often makes use of a third winding of the transformer. The advantages of the switching power supply are that they can operate either as step-up or step-down supplies, by adjusting the number of windings in the primary and secondary of the transformer, and less heat is dissipated by the transistor since it never operates in its linear region. The transistor is either fully off or fully on.
A problem develops in switching power supplies when the load is reduced to almost no load or is removed altogether. The feedback circuit has trouble making the pulses to the switching transistor narrow enough. Device limitations, circuit resistance, parasitic capacitances, loop delays, and the like, all conspire to make a switching power supply literally go wild at minimum loads. The prior art has attempted to correct this situation by a number of ways. One has been to simply put a shunt resistor across the output terminals that will draw the minimum current necessary to keep the output from going wild. The trouble with that is the resistor draws current all the time, it reduces the maximum current available externally, and it produces heat that may be hard to dispose of Zener diodes on the output can draw too much current, and their exact turn-on voltages may not be suitable for particular supplies. Other types of voltage clamps can fail to be properly matched with the voltage regulation point of the basic feedback circuit and so can draw too little or too much current in their attempts to control the output. Subharmonic oscillation of the switching transistor has also been tried. Sometimes called "hiccup mode", subharmonic oscillation can cause audible noise that is objectionable.
What is needed is a switching power supply that provides its own minimum load automatically as a part of the internal feedback control circuit that is deactivated when an external load that exceeds the minimum is connected.